The present invention relates generally to the field of microelectronics, e.g., integrated circuits, and more particularly to compositions and methods for removing photoresist compositions from the surfaces of substrates, e.g., silicon wafers, used in the fabrication of integrated circuits. In its most particular regard, the invention relates to compositions and methods for removing unwanted edge residues of photoresist composition from a wafer which has been spin-coated with photoresist.
In very general terms, the fabrication of integrated circuits involves steps for producing polished silicon wafer substrates, steps for imaging integrated circuit pattern geometries on the various wafer surfaces, and steps for generating the desired pattern on the wafer.
The imaging process involves the use of photoresists applied to the wafer surface. Photoresists are compositions which undergo change in response to light of particular wavelength such that selective exposure of the composition through a suitable patterned mask, followed by development to remove exposed or non-exposed portions of the photoresist as the case may be, leaves on the wafer a pattern of resist which replicates either the positive or negative of the mask pattern, and which thus permits subsequent processing steps (e.g., deposition and growth processes for applying various layers of semiconductive materials to the wafer and etching-masking processes for removal or addition of the deposited or grown layers) to be carried out in the desired selective pattern.
The photoresists used in the imaging process are liquid compositions of organic light-sensitive materials which are either polymers per se or are used along with polymers, dissolved in an organic solvent. Critical to the effectiveness of the selective light exposure and development in forming a resist pattern on the wafer substrate is the initial application of the photoresist composition in a thin layer of essentially uniform thickness on the wafer surface. The coating process of choice in the industry is spin-coating. In this process, the flat circular silicon wafer on which has been deposited a predetermined volume of photoresist composition is subjected to high-speed (e.g., 500 to 6000 rpm) centrifugal whirling to cause the photoresist to spread out evenly as a layer along the wafer surface and such that excess photoresist is spun off the edges of the wafer.
Spin-coating per se is well-known in the art, as is the equipment for such coating and the process conditions employed therein, e.g., to bring about coatings of particular thickness. See, e.g., S. Wolf and R. N. Tauber, "Silicon Processing For The VLSI Era", Volume 1 (Process Technology) (Lattice Press, Sunset Beach, Calif. 1986), incorporated herein by reference, and Skidmore, K., "Applying Photoresist For Optimal Coatings", Semiconductor International, February 1988, pp. 57-62, also incorporated herein by reference.
Despite its widespread use, certain undesirable results also accompany spin-coating. Thus, owing to the surface tension of the resist composition, some of the resist may wick around to and coat the back side edge of the wafer during the spin-coating process. Also, as the spin-coating process progresses, the resist becomes progressively more viscous as solvent evaporates therefrom and resist being spun off the wafer in the latter stages of the process can leave fine whiskers ("stringers") of resist which dry on the edge of the wafer. So too, as the resist continues to dry and increase in viscosity during the spin-coating process, excess resist is less likely to leave the wafer and instead builds up as an edge-bead at the outer reaches of the wafer surface.
These coating-related problems can cause significant difficulties in the overall integrated circuit fabrication process. Resist on the back side of the wafer can be deposited elsewhere and cause contamination, and also prevents the wafer from lying flat on ultraflat surfaces, thereby affecting focus, alignment, planarity, and the like, in subsequent imaging steps. Whiskers on the wafer edges can easily break off in subsequent processing steps and cause particulate contamination in virtually all of the manufacturing equipment. Finally, the edge-bead leads to a distorted surface which can greatly affect focus, alignment, planarity and the like.
The art is aware of the problems associated with residual resist at the edges and sides of the wafer, and generally seeks to overcome them by application at the edge of the wafer of a small stream of a solvent for the resist so as to dissolve and remove the unwanted residue. In many cases, the solvent stream is applied to the backside edge of the wafer and is permitted to wick around by capillary action to the front edges so as to remove backside edge residue, whiskers and edge bead. With certain newer equipment, it is possible to apply the solvent stream from both front and back sides of the wafer simultaneously. In all cases, the object essentially is to remove from the wafer a strip of resist which is adhered to the wafer sides, the back surface outer edges of the wafer, and the outer edges of the front surface of the wafer, which strip typically is at least about 0.5 mm in thickness, and often can be up to 4.0 mm thick. See in this regard, Wolf and Tauber, supra; Skidmore, supra; and N. Durrant and P. Jenkins, "Defect Density Reduction Utilizing Wafer Edge Resist Removal", Microcontamination, April 1985, pp. 45-51, incorporated herein by reference.
Positive-acting photoresists are those most commonly employed in the manufacture of integrated circuits, and typically comprise a binder resin (e.g., a phenol-aldehyde condensation polymer such as a novolak) and a photoactive compound (e.g., an o-quinonediazide) in a suitable organic solvent. For such photoresists, the solvent conventionally employed in removal of the unwanted resist from these edge and side areas after spin-coating (generally termed "edge bead removal (EBR)" processes), is butyl acetate or mixtures of butyl acetate and alcohol. These solvents are not without their own difficulties, however, most notably with respect to their low flash point, and special precautions are required in their handling and use, particularly to avoid spillage during transfer operations.
Generally speaking, the differentiation point between flammability and combustibility is 100.degree. F. (38.degree. C.), TCC, and thus edge bead removal solvents having flash points above 100.degree. F. would be highly desirable. Solvents which have been investigated as possible replacements for the butyl acetate or butyl acetate/alcohol EBR solvents include those which are commonly used in the positive photoresist compositions themselves, such as ethylene glycol monoethyl ether acetate (Cellosolve acetate), propylene glycol monomethyl ether acetate, ethyl 3-ethoxy propionate, and ethyl lactate. Despite desirably low flash points, these solvents do not function well in edge bead removal processes, particularly those involving backside application, in that they typically result in less than 0.25 mm removal.
The primary object of the present invention is to provide a solvent composition for use in edge residue removal processes in integrated circuit and related fabrication sequences, which has a suitably low flash point (i.e., greater than 100.degree. F.) and which efficiently and effectively removes the resist residue in question even when applied in a backside solvent application technique.